Electrical power systems are designed to generate, transmit and distribute electrical energy to various types of electrical loads under varying conditions. Typically, these systems include a variety of power system components such as electrical generators, power transformers, power transmission lines, buses and capacitors, which require protection from abnormal conditions such as electrical short circuits, overloads, frequency excursions, voltage fluctuations, and the like.
To provide such protection power systems typically include a protective device and associated procedures which isolate power system components from the remainder of power system upon detection of an abnormal condition or a fault in, or related to, the protected component. Such protective devices may include different types of protective relays, surge protectors, arc gaps, circuit breakers and reclosers.
Isolation of one or more power system components and/or their associated loads is commonly referred to as “load shedding.” Loads are shed in response to a trip signal transmitted, for example, by a protective relay to a breaker associated with distribution power system element(s) and an associated load(s). The trip signal may be issued as a result of a power source reduction or imbalance detected by the protective relay. In addition to rapid detection, effective power system stability requires fast (breaker) tripping of the correct quantity of load (kW). Because a particular load or loads can change dynamically according to the power system topology and the operating state of the power system, a decision to trip or isolate the load(s) can vary from moment to moment.
In general, load shedding schemes include a number of components designed to monitor the power system and to cause loads to be shed. This ensures that power supplying generators are not overloaded and that a balance of power is supplied by remaining generators in the event of an abnormal fault or condition. An effective load shedding scheme strives to initiate load shedding quickly in response to present power system conditions, to trip the correct amount of load as quickly as possible in order to maintain power system stability, and to avoid unnecessary operations.
While effective in most cases, prior load shedding schemes have limitations under certain conditions. For example, some prior load shedding schemes are based solely on under-frequency detection by stand-alone devices such as protective relays. When a frequency excursion is detected, the protective relay trips its associated breaker. Multiple frequency thresholds may be used in stand-alone devices throughout the power system to shed more loads if the under-frequency condition is not corrected. In addition, time delays may also be used to coordinate load tripping. These approaches, however, do not consider the amount of load (kW) being shed and the importance of each load being shed. Rather, loads are shed and then the power system frequency monitored to determine whether the power system frequency improves. If no improvement is detected, additional loads are shed.
Other prior load shedding schemes utilize a centralized processor (e.g., programmable logic controller or PLC) to make system-wide load shedding decisions. Typically these schemes require a large amount of wiring in order to gather information about the power system (power flows, breaker status, etc.). This approach may be costly in terms of installation, commissioning and maintenance of the system. Further, although PLC-based schemes can be flexible and accommodate large systems, the amount of time needed to process a load shedding algorithm increases proportionally with the complexity of the system, yielding undesirable delays in load shedding in large systems.